Fat

Fat provides richness, moistness, and suppleness. When it melts during cooking, it lubricates the solid particles of flour and sugar and encourages the cookie to spread and thin—a quality that is sometimes desirable, sometimes not. Because butter melts at a lower temperature than margarine or shortening, it gives cookies more time to spread before the protein and starch set. Butter is about 15% water, and is the main or only source of moisture in such low-egg recipes as shortbread and tea cookies.